#define _CRT_SECURE_NO_WARNINGS 1

  struct ListNode {
      int val;
      ListNode *next;
      ListNode() : val(0), next(nullptr) {}
      ListNode(int x) : val(x), next(nullptr) {}
      ListNode(int x, ListNode *next) : val(x), next(next) {}
  };
 
class Solution {
public:
    ListNode* reverseList(ListNode* head)
    {
        if (!head || !head->next) return head;
        ListNode* cur = head->next;
        ListNode* prev = head;
        ListNode* ret = nullptr;
        while (cur)
        {
            if (cur->next == nullptr)
            {
                ret = cur;
            }
            //cout<<"@";
            ListNode* next = cur->next;
            cur->next = prev;
            prev = cur;
            cur = next;
        }
        head->next = nullptr;
        return ret;
    }
};